Sensor for remote detection of objects

ABSTRACT

A sensor for remote detection of objects in a surveillance zone ( 10 ) is preferably intended to be used in an article surveillance zone, which further has at least one transmitter means ( 11, 13 ) and at least one receiver means ( 12, 15 ) for transmitting and receiving, respectively, electromagnetic radio-frequency signals in the surveillance zone, and at least one modulating means ( 16 ) for generating a modulation field in the surveillance zone. The sensor is arranged to transmit an electromagnetic reply signal at the reception of electromagnetic energy from the transmitter means, said reply signal being dependent on the modulation field and being receivable by the receiver means. A magnetic element ( 23 ) is arranged in a sensor body ( 21; 101, 102 ), the magnetic properties of the element being controllable by a magnetic field acting as the modulation field, wherein the amplitude of the reply signal from the sensor is controllable through the magnetic modulation field.

TECHNICAL FIELD

The present invention relates to a sensor for remote detection ofobjects in a surveillance zone, preferably for use in an articlesurveillance system, which further comprises at least one transmittermeans and at least one receiver means for transmitting and receiving,respectively, electromagnetic radio-frequency signals in thesurveillance zone, and at least one modulating means for generating amodulation field in the surveillance zone, said sensor being arranged totransmit an electromagnetic reply signal at the reception ofelectromagnetic energy from said transmitter means, and said replysignal being dependent on said modulation field and being receivable bysaid receiver means.

DESCRIPTION OF THE PRIOR ART

For many years now a large demand for simple and still reliablesurveillance systems for monitoring objects or articles within a givenarea has been noticed in various business and industrial applications. Acommon example is shop antipilferage systems, which are available inmany different kinds. A simple and inexpensive protection is obtained byproviding articles that are especially liable to be stolen (such asclothes) with an antipilferage cassette, which is attached to thearticle. The cassette comprises a liquid substance, such as ink, whichis arranged to discolour the article to make it useless for normal use,if stolen. In U.S. Pat. No. 5,275,122 an exemplary cassette is shown,comprising a base portion, a mounting pin protruding from the baseportion, a pair of liquid-filled glass tubes arranged in the baseportion and acting as an ink ampoule, and a disc mounted below the inkampoule. The mounting pin extends between the two glass tubes and isconnected at a first end to the disc. The article to be protected fromtheft is drawn over the mounting pin in the cassette, for instancethrough a button hole or directly through the fabric, provided that thearticle in question is an article of clothing. The cassette is thenprovided with an upper portion, which is threaded on the mounting pin tosecure the article between the base portion and the upper portion. If anunauthorized person tries to remove the cassette upper portion, the discwill impact the fragile ink ampoule through the mounting pin, whereinthe former will break or otherwise start leaking. The cassette upperportion may only be separated from the base portion by means of acertain device, without causing damage to the ink ampoule.

Strictly mechanical theft protections according to the above have adisadvantage in that even if a potential shoplifter may be discouragedfrom attempting to steal an article (since the article will possibly bedestroyed, if the theft protection is removed), nothing stops theshoplifter from leaving the theft protection untampered on the articleat the actual moment of theft and only later removing the theftprotection at an undisturbed location other than the shop premises. Thisdisadvantage may be eliminated by means of electronic articlesurveillance systems as described below, which may detect the actualshoplifting attempt—i.e. when the shoplifter tries to bring the articleout from the shop premises—and in response thereto generate an alarmsignal so as to alert the shop personnel about the attempted theft.

According to a common type of electronic article surveillance systemseach article is provided with a small label, comprising a thin metalstrip with magnetic properties. On either side of the shop exitarc-shaped magnetic field generating means are arranged for generating amagnetic field in between. When an article, which has been provided withan antipilferage label according to the above, is brought in between thearcs, the metal strip is affected by the magnetic field, and adetectable physical change occurs in the metal strip. Frequently, thefact that an alternating magnetic field will periodically switch themagnetic dipole momentum in the metallic strip is used. Alternatively,the metallic strip may be forced into mechanical resonance, providedthat the material and dimensions of the strip are chosen accordingly.These physical changes are inductively detected by means of the arcs,wherein an attempted theft may be registered. Since the detection ismade by inductive means, antipilferage systems of this kind suffer froma short detection range of a few meters only, requiring theantipilferage arcs to be arranged close to each other and thereby makingthe shop exit narrow and “unfriendly” for the customers.

In addition, various antipilferage systems of a more advanced type arepreviously known. For instance, U.S. Pat. No. 5,030,940 discloses anelectronic article surveillance system. Electronic labels are used formarking and theft-protecting the desired articles. Such an electroniclabel is of a radio-frequency transponder type and comprises forinstance an antenna, a power source such as a battery, and a non-linearcircuit, for instance some kind of semi-conductor diode. Through itsantenna the transponder may receive a first electromagnetic signal of ahigh frequency, which has been transmitted by a transmitter in thesurveillance zone, as well as a second signal of a substantially lowerfrequency, by means of which an electrostatic field is generated in thesurveillance zone. By varying the electrostatic field certain propertiesof the non-linear circuit are influenced, the most important of whichbeing the electric reactance. These variations in reactance areamplified by the power source. The antenna is connected to thenon-linear circuit, and hence a reply signal may be transmitted, whichaccording to the above is composed by the two signals received. When amodified reply signal is detected as described above by a receiver inthe surveillance zone, the system may determine the presence of anarticle within the surveillance zone and provide a suitable alarm signalas a consequence. A drawback of such transponders is that they require aplurality of components as well as a considerable space and a high priceper unit.

Normally, for basic antipilferage applications as described above, it isonly desired to determine the presence of a transponder or sensor in asurveillance zone, but not its identity or exact position in the zone.Such determination, however, is of interest in an adjacent technicalfield, e.g. price labelling of articles. A method and a device for thispurpose are disclosed in WO93/14478. A label acting as a sensor ortransponder is provided with an antenna and at least one electricresonance circuit, comprising inductive as well as capacitive means; aso-called LC-circuit. The resonance circuit is excited toself-oscillation by means of electromagnetic energy transmitted by anexcitation means and received through the antenna of the sensor. Byproviding the label with an amorphous magnetic element and controllingthe permeability of this element by means of an external heterogeneousmagnetic bias field, also the resonance frequency of the resonancecircuit may be controlled, since the change in permeability for theelement will affect the inductive properties of the resonance circuit.Due to the factors above, the frequency of the reply signal transmittedfrom the resonance circuit is dependent on the magnitude and directionof the magnetic bias field in the position, where the sensor happens tobe located. As a consequence, simultaneous detections of a plurality ofidentical sensors present in the surveillance zone are possible, thanksto the reply signals thereof being separated in the frequency domainthrough their different magnetic bias levels. Alternatively, acalculation “backwards” in three dimensions of the position for thesensor is possible by means of the detected frequency value, oncondition that the heterogeneous magnetic bias field is known. Byarranging a plurality of labels and/or amorphous magnetic elements inpredetermined mutual positions a certain code space may be obtained,wherein the reply signal may for instance represent an article numberassigned to the article.

U.S. Pat. No. 5,414,412 relates to a frequency-dividing transponder foruse in an electronic article surveillance system. The transponderresponds to detection of electromagnetic radiation of a firstpredetermined frequency by transmitting electromagnetic radiation of asecond predetermined frequency, which is a frequency-divided quotient ofthe first predetermined frequency. The transponder includes an activestrip of amorphous magnetic material with a transverse uniaxialanisotropy defining a magnetomechanical resonant frequency according tothe dimensions of the strip at the second predetermined frequency, whenmagnetically biased to be within a predetermined magnetic fieldintensity range, so as to respond to excitation by electromagneticradiation of the first predetermined frequency by transmittingelectromagnetic radiation of the second predetermined frequency. Thetransponder further comprises a tripole strip of magnetic material ofsuch coercivity and so disposed in relation to the active strip, as tocreate a magnetomechanical resonance in the active strip at the firstpredetermined frequency, when the active strip is magnetically biased tobe within the predetermined magnetic field intensity range.

SUMMARY OF THE INVENTION

According to the present invention a sensor for remote detection ofobjects in a surveillance zone is proposed, where the communication fromand to the sensor is electromagnetically performed with a long operatingdistance and a wide bandwidth, the output signal from the sensorproviding a satisfactory signal strength as well as being possible tocontrol or modulate in a way, which increases the detection accuracy.Additionally, the object of the invention is to provide a sensor, whichmay be manufactured from a minimum of components and thus at a very lowcost per unit.

The object is achieved by a sensor according to the appended independentpatent claim.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be closer described in the following, reference beingmade to the accompanying drawings, in which

FIG. 1 schematically illustrates an antipilferage system, in which asensor according to the present invention is applied,

FIG. 2 is an enlarged sideview of a sensor according to a preferredembodiment of the invention,

FIG. 3 is an enlarged sectional view of the sensor according to FIG. 2,when viewed from the section III—III in FIG. 2,

FIG. 4 shows a diagram, which illustrates the results from a firsttrial, where some parameters are varied for the preferred embodiment ofthe invention,

FIG. 5 shows a diagram, which illustrates the results from a secondtrial, where some parameters are varied for the preferred embodiment ofthe invention,

FIGS. 6a-6 b show a diagram, which illustrates the results from a thirdtrial, where some parameters are varied for the preferred embodiment ofthe invention,

FIGS. 7a-7 b show a diagram, which illustrates the results from a fourthtrial, where some parameters are varied for the preferred embodiment ofthe invention,

FIGS. 8a-8 b show a diagram, which illustrates the results from a fifthtrial, where some parameters are varied for the preferred embodiment ofthe invention,

FIG. 9 illustrates an alternative to the preferred embodiment of theinvention, illustrating a way of centering an elongated magnetic elementwithin a sensor housing, and

FIG. 10 illustrates an alternative embodiment of the invention.

DETAILED DISCLOSURE OF THE INVENTION

In FIG. 1 an antipilferage system is illustrated, wherein the sensoraccording to the present invention may be applied. A transmitter antenna11 and a receiver antenna 12 are arranged in a surveillance ormonitoring zone 10. The transmitter antenna 11 is operatively connectedto an output stage 13, which in turn is connected to a controller 14.The output stage 13 comprises various conventional driving andamplifying circuits as well as means for generating a high-frequencyelectric current i_(HF), which will alternately run back and forththrough the transmitter antenna 11 when supplied thereto, therebygenerating a high-frequency electromagnetic field at a frequency f_(HF)around the transmitter antenna. As will be described in more detailbelow, this electromagnetic field is used for exciting a transponder orsensor 20 present in the surveillance zone 10, said transponder orsensor transmitting an electromagnetic reply signal to be received bythe receiver antenna 12, in response to the reception of electromagneticenergy from the transmitter antenna 11.

The receiver antenna 12 is operatively connected to an input stage 15,comprising conventional amplifying and signal-processing means, such asbandpass filters and amplifier circuits. Furthermore, the input stage 15is connected to the controller 14 and is arranged to forward a signal,which has been received and processed as described below, to saidcontroller 14.

Hence, the transmitter antenna 11 as well as the receiver antenna 12 arearranged for conventional conversion between an electric high-frequencysignal and an electromagnetic one. Preferably, the antennas areconventional end-fed or center-fed, half-wave whip antennas, but otherconventional antennas may be used just as well.

Additionally, the surveillance zone 10 is provided with a magneticfield-generating means 16, preferably a coil arrangement. In situationswhere the surveillance zone 10 is a monitored shop exit, the coilarrangement is preferably arranged just below the ceiling level orbetween ceiling and roof. This arrangement has an aesthetic advantage inthat the entire coil arrangement is made less noticeable or eveninvisible to the shop customers, thereby consequently providing a higherdegree of freedom when designing an aesthetically appealing shop exit.

Preferably, the magnetic field-generating means 16 comprises an electricconductor, e.g. copper wire, which is wound in one turn or a pluralityof turns around a coil frame. Preferably, the coil arrangement isessentially formed as a rectangle, which is large enough for coveringthe whole of the desired surveillance zone, (e.g. the shop exit) by amagnetic modulation field described below. For this reason thedimensions of the coil arrangement with respect to the length and widththereof are in the order of a few meters.

The coil arrangement 16 is connected to the controller 14 via a drivingstage 17. The driving stage 17 comprises means for generating amodulating current i_(mod), which is fed through the coil for generatinga magnetic modulation field H_(mod) around the coil, the propagation ofthe field covering substantially the entire surveillance zone 10. Themodulating current is given a known variation in amplitude with respectto time according to i_(mod)(t)=f(t). In its most basic form themodulating current corresponds to a pure sine waveform according to

i_(mod)(t)=A sin(2πf_(mod)t),

where A as usual represents the amplitude of the current and f_(mod)represents the frequency thereof, but other more complicated mathematicfunctions are also possible.

When an electric current i is fed through a straight electric conductor,a magnetic field is generated, the field strength H of which islinearity dependent on this current according to

H=i/2πr, where r represents the distance to the conductor, and hence themagnetic modulation field H_(mod) described above will vary inaccordance with the modulating current i_(mod).

In FIGS. 2 and 3 a sensor 20 according to a preferred embodiment of theinvention is shown. A sensor body 21 constitutes a housing for thesensor, but it also takes active part in the actual sensor function, aswill be described in more detail below. The sensor body 21 consists of amaterial with dielectric properties, where the relative dielectricconstant or permittivity ε_(r) is larger than that of air (=1).According to the preferred embodiment the sensor body is formed as acylinder with walls made from glass or a similar material, such as acomposite material consisting of glass fiber and resin, but othermaterials, e.g. plastic material, are possible too. The sensor body 21is sealed at both ends (either directly sealed by melting the glassmaterial, or by separate plugs 22 a, 22 b) according to FIG. 2.

A wire-shaped element 23 of amorphous magnetic material is arrangedinside the sensor body in the longitudinal direction of the latter. Theelement 23 is surrounded by a dielectric liquid, which entirely orpartly fills up the cylinder-shaped cavity inside the sensor body 21.The liquid has a high dielectric constant. In the preferred embodimentthe liquid is ink or another heavily coloured liquid of the kind, whichis already used in conventional antipilferage ampoules, as describedabove in section “Description of the Prior Art”. An advantage of theinvention is in fact that the sensor may have a conventional“discolouring function” in addition to the radio-frequency transponderfunction described below. In such applications the sensor body is formedaccording to the prior art (see for instance U.S. Pat. No. 5,275,122) asan ink-filled glass ampoule with means for breaking the glass, when thesensor is removed from the article by an unauthorized person. A doubleprotection against theft is hereby obtained, since such a sensorarrangement will protect a) the sensor as a whole (if the sensor isremoved from the article, the article will be discoloured) and b) theelectromagnetic sensor function (the wire-shaped element cannot beremoved without breaking the sensor glass and creating a discolourationof the protected article). The liquid-based surroundings of thewire-shaped element have the additional and quite positive effect thatthe signal strength of a reply signal transmitted from the sensor willbe substantially increased (by several dB), as compared to a casewithout liquid.

Even if the liquid according to above has been described as ink oranother coloured liquid, plain water (ε_(r)≈80), an alcohol-based liquid(ε_(r)≈60) or essentially any liquid, preferably with dipole properties,may be used as well.

According to the preferred embodiment of the invention the wire-shapedelement 23 is made from the amorphous cobalt-rich alloyCo_(68.1)Fe_(4.4)Si_(12.5)B₁₅, but other alloys, which fulfill thefunctional demands described below, are possible too. One possiblealternative alloy is Co_(70.5)Fe_(4.5)Si₁₅B₁₀. Both these alloys aremore or less completely free from magnetostriction, meaning that theyare not likely to convert magnetic energy to mechanical energy, therebyavoiding the risk of undesired mechanical resonance phenomena.

The wire-shaped element is electrically conductive, and the physicaldimensions thereof are adjusted for optimum antenna function, asdescribed below. According to the preferred embodiment the diameter ofthe element is 124 μm and the length is 60-65 mm. Furthermore, theamorphous material of the element exhibits magnetic properties, and anessential feature is that the permeability of the material may becontrolled under the influence from an external magnetic field. Thiscontrollable permeability is used in two different ways according to theinvention; for controlling the amplitude of an electromagnetic replysignal from the sensor, and for controlling the frequency of the replysignal.

When a sensor 20 present in the surveillance zone 10 is exposed to theelectromagnetic excitation field with frequency f_(HF) transmitted bythe transmitter antenna 11, the wire-shaped element 23 operates as anantenna. An electric current i_(element) is induced in the wire-shapedelement, provided that the length of the element is adjusted to thefrequency f_(HF)—or rather to the corresponding wavelength—of thereceived high-frequency excitation signal. An element length of 60-65 mmrequires a frequency f_(HF) within the radio-frequency range, preferablybetween 500-900 MHz. Also the dielectric environment (the liquidcombined with the glass material of the sensor body) around thewire-shaped element is important in this respect. The dielectricenvironment causes a reduction in the speed of light c as compared tothe speed of light in vacuum, wherein the length of the antenna isvirtually increased for the sensor. The dielectric environment alsoprovides for an improved reception of an incoming electromagneticsignal, since the dipolar properties of the dielectric environmentamplifies the electric component of the incident electromagnetic field.

Thus, the induced current i_(element) runs back and forth through theelement 23 at a frequency f_(element)=f_(HF). This alternating currentcauses the generation of an electromagnetic field around the element 23,said field propagating through the surveillance zone 10 and reaching thereceiver antenna 12 as a reply signal, the receiver antenna thenreceiving and forwarding the signal to the controller 14 as anindication of the presence of a sensor in the surveillance zone 10.Consequently, the wire-shaped element 23 has the simultaneous functionsof a receiver of the electromagnetic excitation signal as well as atransmitter of the electromagnetic reply signal. However, since thesignals are of the same frequency, they would not be possible toseparate from each other, if the measures below had not been taken.

As described above, by the magnetic modulation field H_(mod) thepermeability μ_(r) of the wire-shaped element material may becontrolled. According to the preferred embodiment H_(mod) variessinusoidally at the frequency f_(mod)=500 Hz. However, other frequenciesare equally possible within a low-frequency range up to at least 1000Hz.

Now, the amplitude of the reply signal may be controlled by the socalled giant-magnetoimpedance effect or skin-depth effect in theamorphous magnetic material of the element. This effect, which has beendescribed in the report “Giant magnetoimpedance and magneto-inductiveeffects in amorphous alloys” in “J. Appl. Phys. Vol. 76, No. 10, Nov.15, 1994” may be summarized according to the simplified model below.

It is a well-known fact that the effective sectional conducting areaA_(eff) in an electric conductor is reduced when the frequency isincreased (so called skin-depth effect). A_(eff), having a strictlycircular shape at DC (the conducting electrons flow everywhere in theconductor), will at high frequencies resemble a ring of a certain widthε. The reason for this is, among others, that eddy currents aregenerated in the interior of the conductor, said eddy currentsrestricting the availability for the conducting electrons. The width eof the effective conductive area A_(eff) may be expressed according to${ɛ = \sqrt{\frac{\rho}{\pi \cdot \mu \cdot f}}},$

where ρ represents the resistivity of the conductive material, μrepresents the permeability thereof, and f represents the frequency.From $R = \frac{\rho}{ɛ}$

 the following is obtained${R = \sqrt{\pi \cdot \mu \cdot f \cdot \rho}},$

i.e. the resistance R is a function of μ and f. Through the μ-dependenceof the resistance the amplitude of the current through the conductorwill change as a function of μ.

If the model above is applied to the wire-shaped element in the sensoraccording to the present invention, it is readily realized that theamplitude of the current i_(element) varies in accordance with thevariations in the magnetic modulation field H_(mod), which according tothe above controls the permeability μ_(r) of the element. As aconsequence, the electromagnetic reply signal transmitted from theelement 23 and the sensor 20 will be constituted by a signal, theamplitude of which is modulated by the frequency f_(mod) and the carrierfrequency of which is f_(HP). This amplitude-modulated signal will thenbe demodulated in a conventional way by the input stage 15. Theadditional information provided by the amplitude-modulation may be usedby the controller for improving the accuracy of the detection; i.e. forminimizing the number of sensors which avoid detection in thesurveillance zone, but also for minimizing the risk of false alarms,when a sensor is located outside the surveillance zone but still repliesto an incoming excitation signal.

As mentioned above, also the frequency of the reply signal may becontrolled by the magnetic modulation field. According to the preferredembodiment of the invention the sensor forms an electric resonancecircuit, which may be functionally compared to the LC-circuit describedin WO93/14478. The dielectric environment around the wire-shaped element23 (the sensor body 21 and/or the liquid 24) provides the capacitiveproperties of the circuit, while the inductive properties are providedby the element 23 itself, which operates as an antenna at the same time.According to the above the element permeability μ_(r) depends on themagnitude of the magnetic modulation field H_(mod), and since L=f(μ_(r))and the resonance frequency f_(res)=F(L), the value of f_(res) willvary, when the sensor is supplied with electromagnetic energy from theexcitation signal, while simultaneously being affected by the magneticmodulation field H_(mod). In other words an electromagnetic reply signalis obtained, the frequency of which is modulated by the frequencyf_(mod) and the carrier frequency of which is f_(HF). As described abovefor the amplitude modulation, a received frequency-modulated replysignal will be demodulated by the input stage 15, before the signal isforwarded to the controller 14.

By performing trials various variations of the sensor according to thedescribed embodiment have been studied. The trial results, which areillustrated as diagrams in FIGS. 4-8b, will now be discussed.

FIG. 4 discloses a diagram from a trial, where the received signalstrength (in dB) was studied, when the length of the wire-shaped elementwas varied insteps of 5 mm for different outer diameters Φ_(ext) andinner diameters Φ_(int), respectively, of the cylinder-shaped sensorbody 21, and for different choices of material for the sensor body. Thelegend in the drawing indicates the variants, to which the differentgraphs are related. The cylinder-length is constantly 75 mm, and thecylinder is completely filled with water. The wire-shaped element ismade from the alloy Co_(68.1)Fe_(4.4)Si_(12.5)B₁₅, and is freelyarranged inside the cylinder 21. The sensor 20 is excited at a constantfrequency f_(HF) within the 500-600 MHz range. One variation consists inthat a thin cylinder is placed inside a thicker cylinder, but thisapparently gives a weaker signal, than if the thicker cylinder was usedalone. The best result (the strongest signal) is achieved when Φ_(ext)=7mm.

FIG. 5 shows the results from a trial, where the length of the elementis constantly 65 mm and where a cylinder with a length of 100 mm isfilled with water up to 65 mm. The outer diameter Φ_(ext) of thecylinder was varied between 3 mm and 24 mm. Apart from this, theconditions correspond to the ones in the previous drawing.

In FIGS. 6a and 6 b the length of the element is constantly 65 mm, andthe length of the cylinder is constantly 100 mm. The water level isvaried inside the cylinder, and so is the cylinder outer diameter. Itappears from the graphs that the best result is achieved for Φ_(ext)=8mm and a water level of 100 mm (completely filled cylinder).

The element length is constantly 65 mm and the cylinder length isconstantly 100 mm also in FIGS. 7a and 7 b. Here the excitationfrequency f_(HM) is varied, and so are the cylinder outer diameter andthe water level. In FIG. 7a the water level is 65 mm, while it is 100 mmin FIG. 7b. It appears from the graphs that the signal reaches optimumat different cylinder outer diameters for different excitationfrequencies.

According to FIG. 8a the element length is constantly 65 mm, thecylinder length is constantly 100 mm, the water level is 70 mm,f_(HF)=638 MHz and f_(mod)=522 Hz. The cylinder outer diameter and thecylinder thickness are varied. The thinnest thickness of material givesthe strongest signal. Furthermore, it is noticed that the signal getsweaker, if the cylinder outer diameter is chosen too large.

The same conditions applies for FIG. 8b as in FIG. 8a, apart from thewater level now being a full 100 mm. The results from the previousdrawing are here confirmed.

From the trial results described above, and from other trial results,the conclusion may be drawn, that an optimum design is achieved, if thefollowing conditions are fulfilled:

the cylinder is completely filled with liquid,

the wire is arranged in the center of the cylinder,

Φ_(ext)≈6-7 mm,

the difference Φ_(xet)−Φ_(int) is as small as possible,

the wire has a maximum length with respect to the length of thecylinder, and

the ε_(r)-value of the liquid is as high as possible.

Furthermore, it appears from the trial results, FIGS. 7a and 7 b inparticular, that the sensor is fully operational within a broadbandfrequency spectrum of at least a few hundred MHz, thereby making thesensor relatively insensible to changes in the excitation frequency.This is a considerable advantage compared to previously known sensortypes, since the manufacturing will be simplified, as the need iseliminated for a precise tuning of the resonance or reply frequency.

Additionally, the sensor has proven to be operational at a distance ofthe order of 10 m, and this too is a substantial advantage compared topreviously known sensor types (cf. the section “Description of the PriorArt” above).

A further advantage of a sensor design according to the above is that itenables a very simple manufacturing process. Basically, one simply hasto provide the glass ampoule 21 with its liquid 24, insert thewire-shaped element 23 and then seal or close the glass ampoule.Apparently, such a sensor may be manufactured at a very low price perunit.

As mentioned above trials have proven, that the reply signal will bestrongest, if the wire-shaped element is firmly mounted in the center ofthe cylinder along the cylinder axis. Such fixed arrangement mayaccording to FIG. 9 be provided by arranging a number of circular discs91 at regular intervals with respect to each other firmly andperpendicularly against the inner surface of the cylinder. A smallopening is made in each disc, wherein the diameter of the opening onlyslightly exceeds the thickness of the wire-shaped element. By insertingthe element through the opening the element may be held in place due tothe frictional engagement between the element and the inside of therespective opening.

Alternatively, amorphous materials of a so-called stress-annealed kindmay be used. This means that the wire-shaped element has been annealedwhen manufactured under the influence from a tensional force along themain axis of the element. Through the annealing the element willmaintain a perfect stiffness and straightness, once the annealing hasfinished. By carefully adjusting the length of such an element so thatthe element will precisely fit between the inner sides of the cylinderends, the element will be maintained in a centered position through thestiffness of the element, as desired.

Furthermore, the wire-shaped element may be made by so-calledflash-annealing, wherein an electric current is fed through the elementfor a certain time, said electric current generating heat as well as amagnetic field around the element suitable for creating optimum domainstructures.

According to a variant of the preferred embodiment of the invention alsothe liquid-based dielectric environment 24 around the wire-shapedelement 23 is given magnetic properties. Preferably, this is achieved byadding a highly fine-grained ferromagnetic or paramagnetic material tothe liquid and suspending the mixture so as to obtain a colloidalsolution. The dissolved magnetic particles are prevented fromsedimentation on the bottom of the cylinder 21 by adding any stabilizerknown per se. Such liquids with magnetic properties are commonlyreferred to as ferrofluids and are well-known within several technicalfields, therefore not requiring any detailed description herein.However, it may be noticed that water-based as well oil-basedferrofluids may be used together with the present invention.

By creating surroundings or an environment for the wire-shaped element23 as described above, where the environment has not only dielectricproperties but also magnetic properties, the following positive effectis obtained. The magnitude of the magnetic modulation field decreasesrapidly with the distance from the coil arrangement 16. If a sensor 20is located in the outmost region of the intended detection zone, thereis an apparent risk of the modulation of the sensor reply signalbecoming too weak for detection. However, the microscopic magneticparticles in the liquid 24 will all be oriented according to theincident magnetic modulation field due to their dipolar magneticproperties (even if the magnetic modulation field is very weak), andeach individual particle contributes to a slight increase in fieldstrength in each respective position. All small contributions are addedto the incident modulation field, thereby considerably increasing thefield strength in the center of the sensor, i.e. where the wire-shapedelement 23 is located. Trials have proven that a weak magneticmodulation field may be amplified at least a hundred times by using aliquid with magnetic properties according to the above.

According to an alternative embodiment of the invention another kind ofmagnetic sensor element is used, namely a multi-layer type of element.Such an element does not have to be wire-shaped, i.e. have a circularcross section, but is preferably given a substantially flat crosssection. The physical effect used according to this embodiment is theso-called magneto-resistance, which is thoroughly described in forinstance the article “Giant magneto-resistance in spin-valvemultilayers”, B. Dieny (“Journal of Magnetism and Magnetic Materials136, 1994, 335-359”). The resistance R of such a material depends on theresistivity ρ_(r) of the material, which in turn depends on amagnetizing field strength H. Thus, R=f(ρ_(r)(H)), wherein a modulationin amplitude according to the one described above may be achieved.

According to a second alternative embodiment the dielectric environmentaround the sensor element 23 is completely made up of a solid material,which forms the sensor body 21 at the same time. According to FIG. 10this sensor body is formed by two rectangular plates 101, 102, which arearranged on top each other with the wire-shaped element 23 arranged inbetween. The dielectric plates may preferably be realized from a highlydielectric material, which today is used in GPS-antennas of so-calledpatch antenna type and which is available from Trans-Tech, Inc.,Adamstown, Md., USA and is described in the publication No. 50040100from this company. Examples of such materials are barium tetratitanateor nickel-aluminium titanate. Alternatively, the sensor body may beformed as one homogeneous element made from the material above, whereina recess will be formed in the center of the sensor body extending alongthe longitudinal axis of the sensor. The wire-shaped element 23 isarranged in this recess. Functionally, the sensor according to thisalternative embodiment corresponds fully to the embodiments describedabove, where the environment consists of a dielectric liquid.

The descriptions above of the various embodiments of the invention areto be taken as embodiment examples only. Other embodiments may deviatefrom the ones described above within the scope of the invention, asdefined in the appended patent claims. For instance, the sensor body 21may not necessarily have to be formed as a cylinder but instead have forinstance a polygonal-shaped cross-section. Furthermore, the sensor bodyand the liquid inside the sensor body are not both required to havedielectric properties. Additionally, the liquid may be omitted, even ifthis is not considered suitable-at the moment with respect to the signalproperties. The sensor element may be given other forms than the onesdescribed above, and the element material may consist of any materialwhich fulfills the functional demands according to the claims anddescription.

What is claimed is:
 1. A sensor for remote detection of objects in asurveillance zone (10), preferably for use in an article surveillancesystem, which further comprises at least one transmitter means (11, 13)and at least one receiver means (12, 15) for transmitting and receiving,respectively, electromagnetic radio-frequency signals in thesurveillance zone, and at least one modulating means (16) for generatinga modulation field in the surveillance zone, said sensor being arrangedto transmit an electromagnetic reply signal at the reception ofelectromagnetic energy from said transmitter means, and said replysignal being dependent on said modulation field and being receivable bysaid receiver means, characterized by a magnetic element (23) arrangedin a sensor body (21; 101, 102) and having a magnetic property, which iscontrollable by a magnetic field acting as said modulation field, sothat the amplitude of the reply signal from the sensor is modulated bysaid magnetic modulation field.
 2. A sensor according to claim 1,characterized in that also the frequency of the reply signal from thesensor is modulated by said magnetic modulation field.
 3. A sensoraccording to claim 1, characterized in that the magnetic element (23) ismade from an amorphous cobalt-rich alloy.
 4. A sensor according to claim3, characterized in that the amorphous alloy is made from any of thealloys Co_(68.1)Fe_(4.4)Si_(12.5)B₁₅ or Co_(70.5)Fe_(4.5)Si₁₅B₁₀.
 5. Asensor according to claim 1, characterized in that the magnetic element(23) is wire-shaped.
 6. A sensor according to claim 1, characterized inthat the wire-shaped magnetic element (23) is manufactured by so-calledflash-annealing.
 7. A sensor according to claim 1, characterized in thatthe magnetic element (23) is contained in an environment (24; 101, 102),consisting of at least one dielectric material, the relative dielectricconstant or permittivity ε_(r) of which is larger than that of air.
 8. Asensor according to claim 7, characterized in that the dielectricenvironment (24) is liquid-based around the magnetic element (23).
 9. Asensor according to claim 8, characterized in that the liquid-basedenvironment (24) around the magnetic element (23) at least partlyconsists of water, ink, alcohol, or any other liquid with a relativedielectric constant or permittivity ε_(r) exceeding 40, or consists ofany combination of such liquids.
 10. A sensor according to claim 8,characterized in that the liquid-based environment (24) is limited by acylinder (21), which is sealed at either ends (22 a, 22 b) and acts as asensor body.
 11. A sensor according to claim 10, characterized in thatthe cylinder (21) comprises glass or a material similar to glass, therelative dielectric constant or permittivity ε_(r) of which is at leastone order of magnitude larger than that of air.
 12. A sensor accordingto claim 10, characterized by the magnetic element (23) being arrangedin the center of the cylinder (21) along its longitudinal axis.
 13. Asensor according to claim 10, characterized in that the magnetic element(23) is arranged in a close vicinity of the inner surface of thecylinder (21) along a direction, which is essentially parallel to thelongitudinal axis of the cylinder.
 14. A sensor according to claim 12,characterized by a plurality of disc-shaped means (91), which arearranged in spatial relationships with respect to each other inside thecylinder (21) in perpendicular connection to the inner surface of thecylinder, and the outer diameter of which correspond to the innerdiameter of the cylinder, an opening being formed in the center of eachdisc-shaped means for receiving and fixing the magnetic element (23).15. A sensor according to claim 12, characterized by the magneticelement (23) being an amorphous wire of a kind, which when manufacturedis heat-treated or annealed under tension in the longitudinal direction.16. A sensor according to claim 8, characterized in that theliquid-based environment (24) of the magnetic element (23) in additioncomprises suspended or dissolved particles with magnetic properties. 17.A sensor according to claim 16, characterized in that the liquid-basedenvironment (24) of the magnetic element (23) at least partly is aferrofluid.
 18. A sensor according to claim 1, characterized by asubstantially homogeneous sensor body, which is made from a soliddielectric material, and a recess formed in the center of the sensorbody in parallel to the longitudinal axis of the sensor, the a magneticelement (23) being arranged in the recess.
 19. A sensor according toclaim 1, characterized by at least two essentially parallel-epipedicplates (101, 102), which are put together for forming the sensor bodyand which are arranged to fix the magnetic element (23) in between andin the longitudinal direction of the sensor.
 20. A sensor according toclaim 1, characterized in that the sensor is arranged to receive anelectromagnetic first signal within a first frequency range Δf₁, toreceive a magnetic second signal with a second frequency range Δf₂,where Δf₁>>Δf₂, and to transmit an electromagnetic reply signal composedby the first signal, the amplitude of which is modulated by the secondsignal.
 21. A sensor according to claim 1, characterized in that thesensor is arranged to receive an electromagnetic first signal within afirst frequency range Δf₁, to receive a magnetic second signal with asecond frequency range Δf₂, where Δf₁>>Δf₂, and to transmit anelectromagnetic reply signal composed by the first signal, the frequencyof which is modulated by the second signal.
 22. A sensor according toclaim 1, characterized by the magnetic element (23) being made from sucha magnetic material, that the amplitude of the electromagnetic replysignal transmitted from the sensor is controllable throughgiant-magneto-impedance effects and/or giant-magnetoresistance effectsin the material, when the sensor is exposed to said magnetic modulationfield in the surveillance zone (10).